How to lower polycount
This answer can only give you the starting points for further research and learning. Google the terms I mention and read up on them, much has been written and there are many tutorials out there if you look for these key words.

For most cases you would be better off cutting it down to maybe 200 vertices or less, assuming you're not right in front of the grenade the whole time.

Generally you will want to take your high-poly version as a reference and build a retopology mesh from it, then transfer the fine details onto the low-resolution model by baking them into a normal map.

The decimate modifier will usually not yield good topology alone, but it is useful for a fast LevelOfDetail generation.